1. Field of the Invention
The present invention relates to a picture coding apparatus for encoding various signals, such as video signals, with a high degree of efficiency by using orthogonal transform.
2. Description of the Related Art
Hitherto, as a technology for encoding video signals with a high degree of efficiency, technology employing orthogonal transform, adaptive quantization and variable-length encoding has been generally known.
In this technology, input video signals are discriminated into frequency components by orthogonal transform such as DCT, and a value (a DCT coefficient) such that those frequency components quantized by a predetermined quantization level, is assigned with a code of an appropriate code length so that picture coding is performed.
In such technology, it is noted that an ordinary subject has relatively small high-frequency components, and that the visual characteristic of the human being is relatively insensitive to high-frequency components in comparison to low-region components. The quantization level with respect to each frequency component discriminated by the above-described orthogonal transform is variably set according to the visual importance and the occurrence probability so that a high-efficiency compression is realized by adaptive quantization such that the frequency components in the low region which are more important are precisely quantized, and the frequency components in the high region which are less important are roughly quantized, and by variable-length encoding such that a short code length is assigned to a DCT coefficient having a high occurrence probability.
The above-described technology, as a technology for realizing high-efficiency compression, is used in digital VTRs for digitizing and recording video signals, or the like. A commercial digital VTR, for example, is capable of compressing video signals (standard definition (SD)) of approximately 160 Mbps in conformity with the current NTSC system to approximately 25 Mbps, i.e., by approximately one-sixth.
Also, since such picture coding using DCT transform is basically an intra-frame encoding, it is particularly suitable for digital VTRs. As described above, in a digital VTR, the above-described encoding is sufficient for a method of compressing video signals (SD) in conformity with the current NTSC system.
However, video signals (high definition (HD)) in conformity with the next television system have a considerably wide signal band, and therefore the amount of data is as large as approximately 1.2 Gbps. Thus, the above-described compression method is not sufficient, which is problematical.